test: fix test-debugger-client.js #5851
                
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Pull Request check-list
make -j8 test(UNIX) orvcbuild test nosign(Windows) pass withthis change (including linting)?
test (or a benchmark) included?
existing APIs, or introduces new ones)?
Affected core subsystem(s)
test, debugger
Description of change
Fix long-broken test-debugger-client by adding missing
\r\n\r\nseparator.
/cc @nodejs/testing